[sdiy] Autorouting (was Eagle)

[Jim Palmer]

firstly, i'm no pro, and almost all my layouts have been digital,
and not super high freq, so it's a bit easier to test for correctness.
i have no doubt there's lots for me to learn, esp in the analog audio 
domain...

also, i'm not talking about taking a 2 hour job and making it a 1/2 hour,
which would make no sense at all because of the learning curve.
i'm talking about taking a few weeks vs a day

> No Auto router can compete with the problem solving capabilities of the 
> human router.

no chess program can beat a human player...
er make that beat a human master...
er make that a grandmaster...
er make that the world champion..
er...

> Also to get the best auto route it requires that routing Rules be VERY 
> well defined.
> This can be quite involved and takes a lot of experience.
>

yes learning the tool is a lot of work and the two i've used each
has had different quirks to learn.  for just making a few small
boards it's not worth the effort, but several complicated layouts
can make up for the time...

> When manually routing, components can be easily shifted to optimize 
> routing. Auto routers don't do this (maybe super expensive ones do)
> and there will be lots of times that tracks will end up circling the board 
> looking for a completion.
>

there are systems with autoplace, and some of these can nudge, sadly not 
eagle...
autoplace seems as frowned on as autorout anyway, and here i think
maybe more correctly so.

> Via count will be at least double with auto route and track lengths will 
> usually be the same.

what's wrong with vias?
i haven't noticed track lengths getting out of hand, will have to check this
on the other hand, in high frequency digital circuits, sometimes you need
traces to be identical in length, and extra length is added just for this 
purpose.
taking care of this by hand would truly suck...


> It can take the strength of Hercules to route some boards and autorouters 
> usually give up long before a good route will materialize.
>

in eagle it will go as long as you tell it to...

> Probably on very complicated digital boards autorouting makes more sense 
> but when you look at commercially made boards,
> they are usually much too well laid out for that to be likely.

maybe you buy better stuff than i do.
i have in my hand a matrox video card that was clearly autorouted.
i think matrox makes pretty good stuff.
i have little doubt i can find numerous similar examples.

> It is possible to do a rough route with the auto router and then manually 
> clean up but depending on your luck this
> can be as labor intensive as routing from scratch by hand.

if this happens, i think either you have a simple layout or the rules can be 
improved.

my take is that cleanup afterwords is just a way to fix some obvious 
shortcomings
when it's easier to do so than to adjust the rules...
the manual routs are better done before invoking the autorouter.

> It does work though sometimes and I have done a few boards this way if I 
> am in a rush.
> It is very important to get power and ground down well and this is often 
> messed up with auto route.
> Fixing this later can be almost impossible or at least difficult.

here is a place where commercial designers get the advantage of multilayer 
pcbs.
i don't make those because they are expensive and harder to solder, esp 
desolder.
but when power is a concern, these traces could be chosen for manual rout...

> Some people do multiple auto routes with different RULE setups to get 
> power and ground down first and then multiple passes to add signal
> traces etc.

i like this idea, i will try it out.

> This can work fairly well but still will never equal a hand routed board 
> for neatness and efficiency.
>

does prettiness really matter?
and efficiency in design time, or ?